Low-temperature tensile properties, deformation and fracture behaviors in the ferrite and austenite duplex stainless steel with various grain sizes

Abstract

Ferrite and austenite duplex stainless steels with different grain sizes ranging from 2 to 11 μm were tensile deformed at room temperature and 77 K, and the effects of grain size on the tensile properties were discussed. The mechanism of improvement of the tensile properties by grain refinement was also discussed based on the observation results of the deformation and fracture behaviors. Specimens having identical phase ratios and chemical compositions in two phases but three different grain sizes ranging from 2 to 11 μm were prepared. At room temperature (RT), the strength increased, but the elongation decreased with decreasing grain size. At 77 K, both the strength and elongation were improved by grain refinement. In the specimen with the smallest grain size, a characteristic increase in the work-hardening rate after satisfying the plastic instability condition occurred during the tensile test at 77 K. Deformation-induced α′ martensite was formed during the tensile test at 77 K, and the volume fraction of α′ martensite increased as the grain size decreased from 11 to 2 μm. This resulted in the highest total strain at 77 K in the fine-grained specimen. The number density and size of voids were decreased by refining the grain size. The distance between two adjacent voids, calculated from the number density and size of voids, assuming a random distribution of voids, increased with grain refinement. It suppressed void coalescence and provided the highest true fracture stress in the fine-grained specimen. The Hall-Petch coefficient of the present duplex stainless steel became larger at 77 K than that at RT, resulting in a higher strength increase by grain refinement at 77 K. It can be concluded that the grain refinement in duplex stainless steel remarkably improves both strength and ductility at low temperatures owing to the high Hall-Petch coefficient, an increase in the volume fraction of deformation-induced α′ martensite, and a decrease in the number density of voids.